Servicing, e.g. maintenance or component replacement of wind turbine drive train components such as a gearbox and/or generator, is in many cases a difficult and expensive activity. Therefore, wind turbine designs should not only be reliable in order to limit servicing activities, but should also be service friendly such that servicing, when necessary, can be performed easily and at low cost.
Nowadays, a lot of effort is done to find solutions for making servicing activities of wind turbines easier and less expensive. One way for obtaining this is to provide a crane on the nacelle structure, thereby eliminating the need for large external cranes. An example hereof is illustrated in EP 1 291 521 which describes a wind turbine comprising a nacelle. The nacelle comprises a rotor and a rotor locking means for preventing rotation of the rotor during maintenance of the nacelle. Furthermore, at a bottom of the nacelle a door is provided for removing wind turbine components, such as gearbox, generator or main shaft, from the nacelle. Before they can be removed from the nacelle through the door, the components first have to be moved backwards, i.e. away from the rotor, in a direction parallel to the axis of the drive train. This can imply that other components present in the nacelle also have to be removed or displaced before the components to be serviced or replaced can be removed from the nacelle.
Another example is illustrated in U.S. Pat. No. 7,735,808 wherein a nacelle frame assembly for a wind turbine and a method for providing heavy lift capability to a wind turbine are provided. The nacelle frame assembly comprises a first hoist substantially permanently mounted along an upper section member of the nacelle frame for performing maintenance operations. The first hoist can move along the upper section member. When having to remove wind turbine parts from the nacelle frame, the first hoist hoists a second hoist and a crane from the workplace ground. Both the second hoist and the crane are then mounted at a rear portion of the nacelle frame. The second hoist and crane are thus temporarily mounted to the nacelle frame. The crane, cooperating with the second hoist, hoists a winch assembly to the nacelle frame until a coupling structure faces the rear portion of the nacelle frame. A carriage structure of the winch assembly is carrying a winch and then becomes removably attached to the rear portion of the nacelle frame. The carriage structure can then be moved towards the parts to be removed from the nacelle frame for lowering these parts through an opening in a lower section member of the nacelle frame.
Similar as described above for EP1 291 521, to be able to lower drive train parts from the nacelle frame described in U.S. Pat. No. 7,735,808, these parts first have to be moved in a direction parallel to the axis of the drive train before they can be removed from the nacelle frame. This again implies that other components in the path of the drive train parts to be replaced also need to be moved or that the nacelle needs to be designed in such a way that there is a wide enough path available to the rear of the wind turbine to remove the large drive train components. This leads to larger, more expensive nacelles.